In general, a library unit functions as a large scale external memory. The library unit is equipped with a number of lockers with storage shelves for housing thousands of recording media cartridges such as magnetic tapes. The unit facilitates large scale automated access operations for writing/reading data to/from each cartridge.
The entire structure of the accessor unit including the hand mechanism (hereinafter "picker section") must be rotatable about the cartridge insertion/ejection direction, because storage shelves are arranged on both sides of the aisle through which the accessor travels. Consequently, the accessor must be equipped with a swivel mechanism to swivel its picker section.
FIGS. 17 and 18 respectively show a plan view and sectional view of a conventional swivel mechanism. As shown in FIG. 18, a shaft 100a is secured to the bottom of a picker section body 100 through a bearing 102 such that the shaft 100a is capable of swiveling on a flat base (swivel base) 101. Notably, the picker section body 100 is swiveled around a vertical axis when driven by a swivel drive motor (swivel motor) 120 via pulleys 120a, 121 and a timing belt 122.
The conventional accessor suffers from several disadvantages. The first disadvantage relates to the operating space requirements for the swivel mechanism. As shown in FIGS. 17 and 18, the configuration of pulleys 120a, 121 and the timing belt 122 of the swivel mechanism dictate that the swivel motor 120 be mounted away from the swiveling axis of the picker section body 100 (shaft 100a). Consequently, the operating space required for the conventional swivel mechanism conflict with the goal of space miniaturization.
A further disadvantage associated with conventional accessors relates to the cartridge delivery mechanism. The cartridge delivery mechanism is provided in a conventional library unit, because conventional hand mechanisms are unable to provide the degree of positioning accuracy required to insert/remove cartridges from the deck. For this reason, a conventional library apparatus is provided with a cartridge delivery mechanism (distinct from the hand mechanism), capable of providing required positioning accuracy. However, in order to downsize and reduce the cost of the library apparatus, it is desirable to omit the cartridge delivery mechanism. This in turn requires the accessor to perform the functions of loading/unloading a cartridge directly to/from the deck without the assistance of the cartridge delivery mechanism.
In order for the accessor to directly perform loading/unloading of a cartridge, the positioning accuracy at the head end of the hand mechanism must be significantly enhanced. However, the construction of a conventional swivel mechanism is designed for ease of maintenance rather than accuracy control. Notably, slack tends to develop, over time, in the timing belt 122, thereby reducing the accuracy control of the swivel mechanism.